Two-Dimensional ITO for Gate-Tunable Optical Absorption

Finely tuned optical thin films and metasurfaces allow for precise control of optical phenomena at the light matter interface. Epsilon Near Zero (ENZ) materials with a particular frequency where their electrical permittivity approaches zero can support ENZ modes which leads to large electric field intensity enhancement within the film and perfect light absorption. The field confinement and perfect absorption can be electrically tuned by biasing the ENZ material in metal/insulator/semiconductor heterostructure via the formation of accumulation/depletion layer of the ENZ materials [1,2]. However, the tunability of the ENZ properties is generally limited in most of the ENZ films thicker than 10 nm because of the small Debye length of the active accumulation/depletion region.<br/><br/>In this work, we utilize two-dimensional ITO ENZ material as active material to develop highly tunable perfect absorber in the NIR regime. Previously studied ENZ thin films containing ITO layer thicknesses in the dozens to hundreds of nanometers range result in weak field confinement and electrical tunability. By utilizing a liquid metal printing technique [3], we are able to fabricate monolayers and bilayers of ITO with atomical layer thicknesses of 1.5 and 3 nm respectively, confirmed by atomic force microscopy. We utilize a reflection and transmission setup to characterize the NIR optical properties of bilayer ITO which as of now, has not been reported by any other group. The optical properties of such mono- and bilayers can be tuned throughout the entire ITO layer rather than confining the change in optical properties to just the ITO interface as shown in previous studies [1,2]. Our numerical simulations reveal strong absorption in the telecommunication wavelength due to the ENZ mode excitation in a heterostructure made of ITO bilayer, thin hafnium oxide spacer, and gold electrode. In addition, post-fabrication tuning of absorption wavelength &gt; 100 nm is predicted in the heterostructure with an applied electrical bias of ± 5 V. This study is an important step in developing ultrathin tunable ENZ devices for linear, nonlinear, and quantum phenomena.<br/><br/>References<br/><br/>[1] A. Anopchenko, L. Tao, C. Arndt, H. W. Lee, “Field-effect tunable and broadband Epsilon-near-zero perfect absorbers with deep subwavelength thickness,” <i>ACS Photonics</i> <b>5</b>, 2631 (2018).<br/>[2] Y.W. Huang <i>et al</i>., “Gate-Tunable Conducting Oxide Metasurfaces,” <i>Nano Letters</i> <i>16</i> (9), 5319-5325 (2016).<br/>[3] R. S. Datta <i>et al.,</i> “Flexible two-dimensional indium tin oxide fabricated using a liquid metal printing technique,” <i>Nature Electronics</i> <b>3, </b>51–58 (2020).